An action for multiplying two input signals v(t), i(t) is essential in order to measure AC mean power, by way of example. In the prior art, an electrodynamometer type of wattmeter has been used for that purpose. But this type of wattmeter comprises a fixed coil and a movable coil and carries out mechanical action for multiplication, and in addition an error is generated due to reactance inside the wattmeter and the error changes according to a power factor of a load or a frequency, so that a compact wattmeter for a broad band is hardly available.
Also there is a wattmeter circuit which uses so-called an analog multiplier IC and outputs an instantaneous voltage e(t) corresponding to v(t).times.i(t), but the circuit is extremely complicated and expensive, and furthermore the following calculation must be carried out separately outside: ##EQU1## if it is necessary to know mean power P=.vertline.V.vertline..vertline.I.vertline. cos .theta..
Conventional power measurement has been carried out as described above, and a compact and simply constructed circuit capable of detecting very small mean power in a broad band, has not been available.